1. Field of the Invention
The present invention is directed to a power supply for electrostatic precipitators of the type having a power electronics stage having a rectifier, intermediate circuit and an inverter, with a transformer connected at the output side, and a high-voltage rectifier, with the transformer and the high-voltage rectifier as well as a device for measured value acquisition situated in the immediate proximity of the electric filter.
2. Description of the Prior Art
Electrostatic precipitators serve for dust removed from gaseous agents in all fields of technology. The gas from which dust is to be removed is conducted between plates that are all grounded and exhibit a spacing of, for example, 600 mm. Wire-shaped spray electrodes that exhibit a highly negative voltage of, for example, 110 kV compared to ground potential are respectively situated therebetween. Due to this high D.C. voltage, the gas molecules are ionized and transfer their charge to dust particles suspended in the gas stream when they strike them. The dust particles become positively charged due to absorbed electrons and migrate to the grounded, negatively charged separation electrode, where they collect and agglomerate into flakes of dust that are stripped from the appertaining plates with vibrators or with a brush mechanism and fall by gravity. This filter method is very efficient but has problems. As a result of the high voltage, voltage arcing regularly occur between the spray electrodes and the plate-shaped separation electrodes. This effect cannot be avoided and is more or less pronounced dependent on the type of dust. The use of electric dust filters is most difficult in steel mill plants since conductive dust particles can greatly shorten the arcing distance between the spray and separation electrodes and thus lead an accumulated occurrence of arcings.
The high-voltage for electrostatic precipitators is usually generated by rectifying the output signal of a high-voltage transformer that is driven by a mains-fed thyristor at the primary side. In the case of a voltage breakdown at the electric filter, a lightning-like discharge arc that can build up. A reliable method for quenching the arc is to wait for the next zero crossing of the primary currant and then blocking the firing pulses of the thyristor for a time interval and resupply the primary side of the high-voltage transformer only thereafter. So that the electric filter remains without high-voltage for an optimally short time span, the inverter should be immediately blocked given a breakdown, so that the current drops as fast as possible and, after the arc has been reliably quenched, can in turn build up as fast as possible by activating the inverter. For this reason, the power electronics of the inverter must be coupled over an optimally short path to the sensors that measure the voltage and the current at the electric filter in order to recognize a breakdown as soon as possible. Given a voltage breakdown at an electric filter, it has been shown that the inductances at the power supply lines can no longer be neglected due to the high currents that thereby occur and can lead to discontinuities in the voltage of the grounded potential of up to 15 kV in the region of the electric filter. Even given employment of coaxial cables, a dependable data transmission from the current and voltage sensors at the electric filter to the drive electronics of the inverter power part is no longer assured in the case of such discontinuities in potential, and the time behavior of the fast disconnect in the event of a voltage breakdown is negatively influenced as a consequence of transmission errors. These disadvantages are alleviated only slightly when the power electronics is arranged in the immediate proximity of the electric filter, since a reliable data transmission is already jeopardized even given distances of a few meters.